Messages in this thread | | | Date | Thu, 28 Sep 2023 12:59:54 -0700 | From | Yury Norov <> | Subject | Re: [PATCH v5 2/5] lib/test_bitmap: add tests for bitmap_{read,write}() |
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On Thu, Sep 28, 2023 at 05:14:55PM +0200, Alexander Potapenko wrote: > On Thu, Sep 28, 2023 at 4:43 PM Yury Norov <yury.norov@gmail.com> wrote: > > > > > > > > On Thu, Sep 28, 2023, 10:20 AM Alexander Potapenko <glider@google.com> wrote: > >> > >> On Wed, Sep 27, 2023 at 9:51 AM David Laight <David.Laight@aculab.com> wrote: > >> > > >> > ... > >> > > Overall, unless allocating and initializing bitmaps with size > >> > > divisible by sizeof(long), most of bitmap.c is undefined behavior, so > >> > > I don't think it makes much sense to specifically test this case here > >> > > (given that we do not extend bitmap_equal() in the patch set). > >> > > >> > Bitmaps are arrays of unsigned long. > >> > Using any of the APIs on anything else is a bug. > >> > So it is always wrong to try to initialise 'a number of bytes'. > >> > The size used in the definition need not be a multiple of 8 (on 64bit) > >> > but the allocated data is always a multiple of 8. > >> > > >> > Any calls to the functions that have a cast of the bitmap > >> > parameter are likely to be buggy. > >> > And yes, there are loads of them, and many are buggy. > >> > >> I got rid of the casts in the bitmap test, but they remain in > >> mtecomp.c, where 16-, 32-, 64-byte buffers allocated by > >> kmem_cache_alloc() are treated as bitmaps: > >> https://lore.kernel.org/linux-arm-kernel/20230922080848.1261487-6-glider@google.com/T/#mdb0d636d2d357f8ffe6ac79cef1145df3440f659 > >> > >> Having them allocated by bitmap_alloc() won't work, because on Android > >> bitmap_alloc() will allocate the buffers from the kmalloc-64 cache, > >> defeating the purpose of the compression. > >> > >> Would it be better to extend the bitmap.h API so that it is possible > >> to allocate from a kmem cache (which would in turn require > >> bitmap_kmem_cache_create() to ensure the alignment requirements)? > > > > > > So all that is wrong then. Bad on me, I'd spend more time looking into your driver code... > > > > We already have bitmap_(from,to)_u(64,32), > > And you can use them. For 16-bit you have to add helpers yourself. But it's not a rocket science. > > > > So e.g. for compressing something into a 16-byte buffer using bitmaps > I'd need to: > > 1) Allocate the buffer: buf = kmem_cache_alloc(...) > 2) Allocate the bitmap: bitmap = bitmap_alloc(16*8, ...) > 3) Fill the bitmap: mte_compress_to_buf(..., bitmap, 16) > 4) Copy the bitmap contents to the buffer: bitmap_to_arr64(buf, bitmap, 16*8) > 5) Deallocate the bitmap: bitmap_free(bitmap) > > instead of: > > buf = kmem_cache_alloc(...) > mte_compress_to_buf(..., (unsigned long *)buf, 16) > > , correct? > > Given that the buffer contents are opaque and its size is aligned on 8 > bytes, could it be possible to somehow adopt the `buf` pointer > instead?
I didn't find an explicit typecasting where you're using mte_compress_to_buf(), but now after hard 2nd look I see...
Firstly, now that in the documentation you are explicitly describing the return value of mte_compress() as 64-bit frame, the right way to go would be declaring the function as: u64 mte_compress(u8 *tags).
And the general pattern should be like this:
unsigned long mte_compress(u8 *tags) { DECLARE_BITMAP(tmp, MTECOMP_CACHES_MAXBITS); void *storage; ... if (alloc_size < MTE_PAGE_TAG_STORAGE) { storage = kmem_cache_alloc(cache, GFP_KERNEL); mte_compress_to_buf(r_len, r_tags, r_sizes, tmp, alloc_size); switch (alloc_size) { case 16: bitmap_to_arr16(storage, tmp, 16); break; case 32: bitmap_to_arr32(storage, tmp, 32); break; case 64: bitmap_to_arr64(storage, tmp, 64); break; default: pr_err("error\n"); } result = ((u64)storage | cache_id) & MTE_HANDLE_MASK; goto ret; } ... } Yeah, it looks cumbersome, but this is the right way to go if you need a reliable BE-compatible driver. I think it will be less scary if you wrap the switch with a helper, and/or move it inside mte_compress_to_buf(), so that the mte_compress will stay unchanged.
Anyways, hope the above helped.
Thanks, Yury
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